Devices connected to the Internet are called nodes . Nodes that are computers are called hosts . Each node or host is identified by at least one unique number called an Internet address or an IP address. Most current IP addresses are four bytes long; these are referred to as IPv4 addresses. However, a small but growing number of IP addresses are 16 bytes long; these are called IPv6 addresses. (4 and 6 refer to the version of the Internet Protocol, not the number of the bytes in the address.) Both IPv4 and IPv6 addresses are ordered sequences of bytes, like an array. They aren’t numbers, and they aren’t ordered in any predictable or useful sense.

An IPv4 address is normally written as four unsigned bytes, each ranging from 0 to 255, with the most significant byte first. Bytes are separated by periods for the convenience of human eyes. For example, the address for hermes.oit.unc.edu is 152.2.21.2. This is called the dotted quad format.